Method for the refining of a carbonaceous metal melt

ABSTRACT

The final phase of carbonaceous metal melt refining operation is carried out with a deficiency of oxygen in the furnace space in order to reduce the wear of the furnace lining without decreasing the content of iron oxide in the slag.

United States Patent Inventors Appl. No.

Priority Hans Karl Gustav Sandberg;

Erik Axel Bengtsson, both of Borlange, Sweden Apr. 22, 1969 Nov. 2, 1971 Stora Kopparbergs Bergslags Aktiebolag Falun, Sweden Apr. 25, 1968 Sweden METHOD FOR THE REFINING OF A CARBONACEOUS METAL MELT 9 Claims, 1 Drawing Fig.

US. Cl 75/60 Int. Cl C2lc 5/32 Field of Search 75/60 References Cited UNITED STATES PATENTS 9/1958 Kalling et a1 9/1959 Kalling et a1.. 3/l96l Graefet a1. 8/1963 Bogdandy et a1. 4/1967 Decamps 4/1967 Johansson et al FOREIGN PATENTS 8/1952 Great Britain 5/1965 Great Britain....

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-G. K. White Attorney-Curtis, Morris & Safford ABSTRACT: The final phase of carbonaceous metal melt refining operation is carried out with a deficiency of oxygen in the furnace space in order to reduce the wear of the furnace lining without decreasing the content of iron oxide in the slag.

INVENTORS HANS SANDBERG ERIK AXEL BENGTSSON ta mr ATTORNEYS METHOD FOR THE REFINING OF A CARBONACEO US METAL MELT The present invention relates to the refining of a carbonaceous metal melt, particularly, but not exclusively, molten pig iron and other carbonaceous alloyed or unalloyed ferrous materials, the refining being carried out in the presence of slag-forming substances, for example lime material, by blowing with an oxidizing gas, for example oxygen, against the melt in a rotary furnace having a fireproof lining, substantially all the carbon monoxide being burned with the oxidizing gas supplied and the heat thus obtained being transferred to the melt by rotation of the furnace. Due to the combustion of the carbon monoxide the process has very good heat economy which can be used to melt scrap or for reduction of ore directly in the refining process. One disadvantage with such refining processes, however, is that wear on the lining is greater than with other conventional refining processes.

It is known that, for example, with the refining of pig iron to steel in a rotary furnace mentioned above, a great deal of the wear on the lining occurs during the last stage of the process. The refining is usually carried out in two phases. In a first phase, when most of the carbon is removed, the slag is solid. When more than 80 percent of the carbon has been removed, the slag melts and is tapped off. After this a second state is initiated where the remaining carbon in the melt, which now has a low carbon content, is removed in the presence of a liquid slag having high iron oxide content. Approximately half the wear. of the lining occurs in the second phase although this phase takes a much shorter a much shorter time than the first phase. Measurements of the wear of the lining have shown that the lining becomes worn four times more quickly in the second phase than in the first phase. It is considered that this high rate of wear is due to the percentage of iron oxide in the slag.

When pig iron having a high content of phosphorous (over 1 percent P) is to be refined, the first phase may be completed with a carbon content of 0.5 percent and a phosphorous content of 0.1 percent in the melt. When pig iron poor in phosphorous (below 0.5 percent P) is used, the first phase may be completed with lower carbon contents, towards 0.1 percent. The slag is possibly not tapped off between the two phases in the latter case. In the final phase a high content of iron oxide is desired, inter alia, to obtain a low percentage of phosphorous in the steel finally produced.

It has now proved possible to considerably reduce the wear of the lining in the final phase of the refining without decreasing the content of iron oxide in the slag. According to the invention the final phase of the refining when the carbon content in the melt is low, preferably below 0.5 percent by weight, is carried out with a deficiency of oxygen in the furnace atmosphere. This expression means that there is not enough oxygen to burn all carbon monoxide to carbon dioxide. Because of the high temperature the burning is almost instantaneous and consequently the furnace atmosphere will contain only carbon monoxide and carbon dioxide except in the immediate vicinity of the oxygen jet from the lance. The percentage of carbon monoxide in the furnace atmosphere should preferably be above 50 percent by volume. The best result is obtained if the volumetric or molecular ratio between CO and CO, in the furnace atmosphere is 6:1 or greater. Simultaneously the iron oxide content of the slag is preserved and is preferably above percent by weight Fe.

in spite of this quantity of iron oxide in the slag a decrease in the wear of the lining has been observed in the final phase of more than 50 percent with the use of the invention.

The oxygen deficiency in the furnace atmosphere, while retaining the iron oxide content, can be effected by reducing the supply of oxygen gas and simultaneously increasing the supply of iron oxide, for example ore. This will, however, result in a delayed refining and loss of temperature. A better method instead, is to blow the oxidizing gas substantially at right angles to the surface of the metal melt. Furthermore, the distance between the nozzle through which the oxidizing gas is blown and the surface of the melt is short, preferably less than 0.5 m. These measures also ensure that the iron oxide content of the slag is preserved.

It is possible to diminish the splashing on the special lance for blowing at right angles to surface by having the rotary furnace stationary during the final phase and thereby diminishing the possibility of damaging the lance. According to another embodiment of the invention, the furnace is, when stationary, raised to its vertical position, assuming that the rotary furnace is closed at one end.

After completed refining the melt and slag can be mixed, possibly after the addition of more lime, by rotating the furnace, no oxidizing gas, or very little, being blown in. This is particularly important if the final phase is carried out in a stationary furnace, as the mixing in the stationary furnace often is insufficient.

The following steps were taken when using the invention for the refining of Thomas pig iron in a rotary furnace of the KALDO type:

When the furnace had been charged with 30 tons of pig iron containing 0.3 percent Si, 1.8 percent P and 0.4 percent Mn, 3.8 tons of lime and 4.0 tons of iron ore were added. Oxygen was then blown into the furnace above the melt at a rate of 2.0 Nm./min./ton in the normal manner, i.e. horizontally or obliquely to the surface of the melt, while the furnace was rotated at 30 r.p.m. The pig iron was refined in this manner, constantly burning substantially all the carbon monoxide formed. When the refining started to abate the slag melted, which occurred 28 minutes after the blowing had started. This was interrupted and the slag, now containing substantially all the phosphorous from the pig iron, was tapped off. The carbon content of the melt was then 0.5 percent and the phosphorous content 0.1 percent. The wear of the lining in the furnace was measured as about 3 mm. on a representative surface. 0.3 tons of lime and 0.3 tons iron ore were then added and the blowing continued with a different nozzle directing the flow of oxygen gas almost perpendicular to the surface of the melt. The furnace was rotating at the same speed as before. The supply of oxygen gas was then 1.6 Nm. /min./ton. After 4 minutes the blowing was stopped and the steel could be tapped with a carbon content of 0.1 percent and 0.02 percent phosphorous. The wear of the lining during this period was 1.0 mm., compared with 1.9 mm. when the blowing is carried out conventionally.

In order to elucidate how the blowing with oxygen can be controlled in accordance with this invention, reference was made to the accompanying drawing showing an apparatus for refining pig iron with oxygen gas in a rotary furnace. This embodiment, however, should only be considered as an example.

The drawing shows a vertical section of a sloping rotary furnace 1 adapted for refining pig iron or other metals'and provided with a refractory lining 2. The furnace is surrounded by a frame 3 provided with treads 4 for driving rollers 5 and idle rollers 6, the former being driven by a motor 7. Guiding rollers 8 are provided for limiting the axial movement of the furnace. The rollers are journaled in the cradle 9 which is mounted in bearings 10 by pivoting shafts 11 so that the whole furnace can be tipped in different positions suitable for charging and tapping as well as for the heat treatment. The furnace has a single opening 12 located at the center of one end wall. An exhaust gas hood 13 is located in front of this opening when the furnace is in position for the heat treatment at an inclination of eg 15 to 20. The hood is provided with a cooling jacket 14 having water inlet 15 and outlet 16. The exhaust hood is pivoted by about a vertical shaft 18 secured to the base 17 so that it can be swung away and make the furnace accessible for charging, sampling and similar operations.

Slideable mounted in the hood are two tuyeres, one 19 for the ordinary refining and one 20 for the end step of the refining. Both tuyeres are water cooled as indicated by water inlets 21 and 23 and water outlets 22 and 24, respectively. Gas rich in oxygen is supplied to the pipe 26 to tuyere 19 through the flexible hose 28, and to the pipe 27 of tuyere 20 through the flexible hose 29.

in the figure both tuyeres are shown in their foremost position but when refining metallic melt according to the invention the refining is started with the tuyere 20 retracted out of the furnace and blowing oxygen only through tuyere 19 until most of the carbon in the melt has be oxidized. When the carbon content of the melt is low and preferably only 0.5 percent above the intended end point of the carbon content the tuyere 19 is retracted and oxygen is instead blown through tuyere 20, which is moved into the position shown in the figure. The mouth of the pipe 27 on the tuyere 20 is directed vertically against the surface of the melt and by restricting the supply of oxygen through the hose 29 the content of carbon monoxide in the atmosphere of the furnace is kept above 50 percent by volume.

What is claimed is:

l. A method for refining a carbonaceous metal melt in the presence of a slag-forming substance by blowing with an oxidizing gas in a rotary furnace comprising a first step in which the metal melt is refined to a carbon content of about 0.5 percent by weight and substantially all the carbon monoxide formed during this refining is converted to carbon dioxide within the furnace chamber, and a second step in which the refining is carried out with a deficiency of oxygen in the furnace atmosphere corresponding to a carbon monoxide content above 50 percent by volume of the atmosphere.

2. The method as defined in claim 1, and wherein the molecular ratio of COzCO, in the furnace atmosphere in the final phase of the refining is at least 6:1.

3. The method as defined in claim 1, and wherein iron oxide in the slag is maintained during the second step.

4. The method as defined in claim 3, and wherein iron oxide in the slag is maintained during the second step in an amount of at least 10 percent by weight on basis of Fe.

5. The method as defined in claim 1, and wherein the oxidizing gas is blown during the second step substantially at right angles to the surface of the melt.

6. The method as defined in claim I, and wherein the oxidizing gas is blown during the final phase against the surface of the melt through a nozzle situated near the surface.

7. The method as defined in claim 6, and wherein said nozzle is less than 0.5 meters from said surface of said melt.

8. The method as defined in claim 1, and wherein after completion of the refining, the melt and slag are mixed by rotation of the furnace and the ratio ot'COzCO, in said furnace is at least 6: 1.

9. The method as defined in claim 8, and wherein the melt and slag are mixed after addition of lime. 

2. The method as defined in claim 1, and wherein the molecular ratio of CO:CO2 in the furnace atmosphere in the final phase of the refining is at least 6:1.
 3. The method as defined in claim 1, and wherein iron oxide in the slag is maintained during the second step.
 4. The method as defined in claim 3, and wherein iron oxide in the slag is maintained during the second step in an amount of at least 10 percent by weight on basis of Fe.
 5. The method as defined in claim 1, and wherein the oxidizing gas is blown during the second step substantially at right angles to the surface of the melt.
 6. The method as defined in claim 1, and wherein the oxidizing gas is blown during the final phase against the surface of the melt through a nozzle situatEd near the surface.
 7. The method as defined in claim 6, and wherein said nozzle is less than 0.5 meters from said surface of said melt.
 8. The method as defined in claim 1, and wherein after completion of the refining, the melt and slag are mixed by rotation of the furnace and the ratio of CO:CO2 in said furnace is at least 6:1.
 9. The method as defined in claim 8, and wherein the melt and slag are mixed after addition of lime. 